This invention generally relates to absorption spectroscopy and more particularly relates to a micro spectrometer, which provides a profile of the transmission percentage versus wavelength for relatively small volume or low concentration samples.
Industry experts agree that the emerging requirements for biological and chemical warfare necessitates a small, portable device capable of detecting trace amounts of various chemicals in air. In some cases, the concentration in air of various biological and chemical reagents of interest may be approximately 0.1 to 10 ppm or less.
Conventionally, chemical detection may be accomplished using absorption spectroscopy. Spectroscopy is used to identify various unknown substances by reading spectroscopic patterns. Absorption spectroscopy relies on the consistent absorption or fluorescence by various compounds at specific wavelengths of light that produce a consistent pattern identifying the substance.
In practice, vibration bands within a molecule selectively absorb wavelengths corresponding to the energy level of the vibration bands. Thus, the absorption spectrum of a chemical compound will typically comprise a series of absorption bands, which are fixed with respect to wavelength and intensity. In practice, each of the absorption bands originates from an interatomic bond within the molecule.
For example, FIG. 1 depicts a two-atom compound Axe2x80x94B having an absorption spectrum associated with the stretching of the bond Axe2x80x94B. In this example the absorption frequency is given by Eq. 1 as follows:                     v        =                                            (                              1                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      c                    s                                                              )                        ⁡                          [                                                k                  f                                ⁢                                                      [                                                                  M                        A                                            +                                              M                        B                                                              ]                                                                              M                      A                                        ⁢                                          M                      B                                                                                  ]                                            1            2                                              Eq        .                  xe2x80x83                ⁢                  (          1          )                    
where xcexd is the wavenumber in cmxe2x88x921 of the absorption band [xcexd (cmxe2x88x921)=104/xcex, xcex is the wavelength, where xcexd and xcex in cmxe2x88x921 and xcexcm, respectively], cs, is the velocity of light (approximately 3xc3x971010 cm/s), MA and MB are the masses of the two atoms (A and B respectively), and kf is the spring constant of the bond. Thus, as the bond stiffness increases, the wavelength of the corresponding absorption band decreases. In general, a molecule with N atoms will comprise 3N-6 normal absorption bands, each with a distinct absorption frequency.
The intensity of the absorption band is governed by a number of factors as provided in Eq. 2:                                                         ∫              band                              xe2x80x83                                      ⁢                                          ln                ⁡                                  (                                                            I                      o                                        I                                    )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                v                                              =                      xe2x80x83                    ⁢                                    A              s                        ⁢            cb                          ⁢                  
                ⁢                              A            s                    =                      xe2x80x83                    ⁢                                    [                                                8                  ⁢                                      π                    2                                    ⁢                                      N                    A                                                                    3                  ⁢                  ℏ                  ⁢                                      xe2x80x83                                    ⁢                                      c                    s                                                              ]                        ⁢            v            ⁢                                          "LeftBracketingBar"                                                      ∂                    μ                                                        ∂                                          Q                      s                                                                      "RightBracketingBar"                            2                                                          Eq        .                  xe2x80x83                ⁢                  (          2          )                    
where, Io and I are the intensities of the incident and transmitted light, respectively, c is the concentration (moles/liter), b is the path length (cm), h is Planck""s constant (approximately 6.63xc3x9710xe2x88x9234), NA is Avogadro""s umber (6.23xc3x971023) and xcexc is the dipole moment, where the integration is taken over the entire absorption band and the partial derivative refers to the derivative in normal coordinate space.
Thus, Eq. 2 effectively states that the two non-atomic factors that affect the intensity of the absorption band are the sample concentration and the optical path length. However, the signal to noise ratios in typical absorption signals is relatively low making it difficult to provide instrumentation capable of detecting relatively weak absorption bands.
For example, absorption spectroscopy has historically been performed in the continuous wave spectroscopy (CW-SPEC) mode. In this instance a sample of interest is irradiated with white light and the transmitted light is spatially resolved into separate wavelengths (e.g. by the use of a Fiber Bragg grating, or by the use of dispersive prisms). A photodetector may measure the separate wavelengths providing the transmission spectrum. Alternately, the incident light is temporally resolved into different wavelengths (e.g., by the use of a rotating prism and a white light source), and the transmitted light is measured by a photodetector. This approach is limited in sensitivity because most of the energy generated by the light source is discarded by the dispersive mechanisms used.
More recently Fourier Transform absorption spectroscopy, (FT-SPEC), has been used to improve the measurement sensitivity by continuously detecting all the wavelengths. This technique was enabled by the development of the Michelson interferometer as illustrated in FIG. 2. Generally a Michelson interferometer may comprise a fixed mirror 34 and a moving mirror 33. In practice a light source 31 may be used to generate white light, which is collimated onto a sample 39, and then onto a beamsplitter 32.
The beamsplitter 34 divides the incident beam into two separate optical beams, one of which is incident on the moving mirror 33, and the other is incident on the fixed mirror 34. In practice each of the mirror are reflective at the wavelengths of interest and reflect the incident beams back to the beamsplitter where they are combined and forwarded to a detector 35. In operation, the lengths of the two optical paths are different so that the intensity of the recombined light varies in accordance with the constructive and destructive interference of the two beams as given by Eq. 3.                               I          x                =                              ∫            v                          xe2x80x83                                ⁢                                                    A                v                            ⁡                              (                                  1                  +                                      cos                    ⁢                                          {                                              2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                        vx                                            }                                                                      )                                      ⁢                          xe2x80x83                        ⁢                          ⅆ              v                                                          Eq        .                  xe2x80x83                ⁢                  (          3          )                    
where Axcexd is the intensity of the incident, unmodulated light, and x is the path length difference. Eq. 3 represents a Fourier transform of the intensity of the incident beam Axcexd so that the intensity of the incident beam, Axcexd, may be estimated from the inverse FFT of the intensity of the recombined optical beam Ix. Typically a calibration interferometer having a monochromatic light source 36, and a white light source 37 may be used to calibrate the Michelson interferometer as illustrated in FIG. 3.
This technique is called Fourier transform spectroscopy because the transmission spectrum of the sample is obtained as the inverse Fourier transform of the raw detector output. Fourier transform spectroscopy typically provides improved sensitivity compared to CW-SPEC instruments (with similar detectors and light source instrumentation).
Both CW-SPEC and FT-SPEC instrumentation are widely available and used in the academia and industry. Unfortunately, both CW-SPEC and FT-SPEC are unsuitable for detection of samples in the small size limit (i.e. when the concentration in air is approximately 0.1 ppm, or the total sample size is approximately 1 nano-liter or less). The detection limit in absorption spectroscopy is given by the minimally detectable absorption determined by the signal-to-noise ratio of the measurement instrumentation.
Consider an optical radiation at a frequency v and power P incident on a collection of N molecules (the sample). If the N molecules absorb a fraction of the incident power xcex7 (xcex7 less than  less than 1), and the rest is transmitted to a detector of quantum efficiency xcex2, and the incident power is below the saturation intensity, then the detector current is given by Eq. 4.                               I          D                =                                            P              ⁡                              (                                  1                  -                  η                                )                                      ⁢            e            ⁢                          xe2x80x83                        ⁢            β                                ℏ            ⁢                          xe2x80x83                        ⁢            v                                              Eq        .                  xe2x80x83                ⁢                  (          4          )                    
and the signal Is (the change in detector current due to absorption) is given by Eq. 5 as follows:                               I          S                =                              P            ⁢                          xe2x80x83                        ⁢            η            ⁢                          xe2x80x83                        ⁢            e            ⁢                          xe2x80x83                        ⁢            β                                ℏ            ⁢                          xe2x80x83                        ⁢            v                                              Eq        .                  xe2x80x83                ⁢                  (          5          )                    
If absorption is due to an atomic transition with resonance frequency xcexdo, then the fraction of the absorbed incident power xcex7 can be written as Eq. 6.                     η        =                              N            ⁢                          xe2x80x83                        ⁢            σ                    A                                    Eq        .                  xe2x80x83                ⁢                  (          6          )                    
where a "sgr"="sgr"oK, "sgr"o is the resonant cross section of the particular transition of interest; and K is a factor less than one that represents the reduction in cross section due to the radiative decay of the transitions into atomic states other than the lower state of the transitions of interest, Doppler broadening, collisional broadening, detuning of the source away from xcexdo and the finite bandwidth of the radiation source.
Thus, the signal to noise ratio is given by Eq. 7 as follows:                               S          N                =                                            I              S                                      i              n                                =                                    N              ⁢                              xe2x80x83                            ⁢                              σ                o                            ⁢              Ke              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢                              P                /                A                            ⁢                              xe2x80x83                            ⁢              ℏ              ⁢                              xe2x80x83                            ⁢              v                                                      [                                                      i                    d                    2                                    +                                      i                    t                    2                                    +                                                            2                      ⁢                                              e                        2                                            ⁢                      β                      ⁢                                              xe2x80x83                                            ⁢                      P                      ⁢                                              xe2x80x83                                            ⁢                      Δ                      ⁢                                              xe2x80x83                                            ⁢                      v                                                              ℏ                      ⁢                                              xe2x80x83                                            ⁢                                              v                        o                                                                                            ]                                            1                2                                                                        Eq        .                  xe2x80x83                ⁢                  (          7          )                    
In Eq. 7, the three statistically independent sources of noise in the system include the detector noise (id), the radiation source technical noise (it) and the shot noise in the detection system given by:       i    sh    2    =            2      ⁢              eI        D            ⁢      Δ      ⁢              xe2x80x83            ⁢      v        =                  2        ⁢                  e          2                ⁢        β        ⁢                  xe2x80x83                ⁢        P        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        v                    ℏ        ⁢                  xe2x80x83                ⁢                  v          o                    
Thus, the S/N ratio can be increased by increasing the source power P. This approach is limited to power levels below the saturation limit governed by:                               σ          o                ⁢        KP                    A        ⁢                  xe2x80x83                ⁢        ℏ        ⁢                  xe2x80x83                ⁢                  v          o                      ≅          γ      2        ;
where 1/xcex3 is the radiative lifetime of the upper transition state. In practice the numerator in Eq. 7 remains relatively constant but the shot noise in the detection system ish continues to increase for power levels above the saturation limit, thereby reducing the S/N ratio.
Alternatively the signal to noise ratio may be improved by increasing the quantum efficiency xcex2, of the detector as well the detection bandwidth xcex94xcexd or by reducing the detector noise id or the radiation source technical noise. The detector noise id is generally given by:
id=4kBTxcex94xcexd/RD,
and can be reduced by reducing the absolute temperature T. The radiation source technical noise can be reduced by frequency modulation schemes.
In addition the signal to noise ratio may also be improved working with transitions with small radiative upper state lifetimes (and thus a higher saturation limit), a large cross section, and K close to 1 or by increasing the number of molecules N in the sample.
The present invention provides instrumentation for chemical detection. In one aspect the present invention enhances the sample-electromagnetic wave interaction by the use of an absorption enhancement signal apparatus, such as, for example, a Fabry-Perot etalon cavity.
In an exemplary embodiment a sample is placed inside a Fabry-Perot etalon cavity, which is then irradiated with a broadband white light source, and tuned such that one of the transmission peaks of the etalon scans across an absorption band in the sample. The power spectrum of the transmitted profiles are measured with an optical spectrum analyzer during the etalon tuning operation, and the height of the transmission peak measured as a function of its wavelength and etalon tuning. From this, an absorption spectrum of the sample is reconstructed.
In another aspect of the present invention an absorption spectrometer includes an absorption signal generator having a cavity for receiving a sample, wherein an input optical beam traverses a multi-bounce optical path across the sample from an input of the cavity to an output of the cavity and an optical spectrum analyzer coupled to the output of the cavity measures the intensity of the cavity output as a function of wavelength